helper = $this->getMockBuilder(\Magento\Wishlist\Helper\Data::class) ->disableOriginalConstructor() ->getMock(); $this->observer = new Observer($this->helper); } public function testExecute() { $event = $this->getMockBuilder(\Magento\Framework\Event\Observer::class) ->disableOriginalConstructor() ->getMock(); /** @var $event \Magento\Framework\Event\Observer */ $this->helper->expects($this->once()) ->method('calculate'); $this->observer->execute($event); } }